Kinetics of PSMA PET signal after radiotherapy in prostate cancer lesions: A single-center retrospective study

Abstract

Purpose: To evaluate the kinetics of prostate-specific membrane antigen (PSMA) PET uptake in irradiated lesions using serial PSMA PET/CT scans.

Methods: Patients with prostate cancer who underwent ⁶⁸Ga-PSMA-11 PET/CT before (PET1) and after radiotherapy (PET2) were retrospectively included. Percentage changes in SUVmax (ΔSUVmax) of the irradiated lesion were measured. The presence of residual uptake was visually assessed on PET2. When available, follow-up imaging was used for lesion validation. Morphologic or uptake disappearance on follow-up scans was defined as loco-regional complete response (L-CR). Clinical and PET characteristics were compared between lesions with and without residual uptake. An optimal timing for response assessment was calculated by receiver-operating-curve analysis.

Results: Eighty-nine patients with 217 irradiated lesions (106 lymph nodes, 85 bone, 21 prostate/prostate bed) receiving ablative radiotherapy were included. Lesion uptake was lower at later time points and was lowest at 9-12 months after radiotherapy. Sixty-eight lesions showed residual uptake on PET2. Residual uptake was more common in lesions imaged at an earlier time point after radiotherapy (median: 7.9 vs. 13.0 months, p = 0.001), lesions in the prostate/prostate bed (p < 0.001), and lesions with higher baseline SUVmax (p = 0.001). Thirty-one residual uptake-positive lesions had available follow-up imaging, of which 24 lesions were confirmed to be L-CR. Risk factors for not achieving L-CR were lesions with prolonged uptake (p = 0.002) and those in the prostate/prostate bed (p = 0.003). The optimal time point for predicting L-CR was 8.6 months.

Conclusions: Timing and tumor site affect the PSMA PET signal after radiotherapy, and should be considered when assessing response on post-radiotherapy PSMA PET.

Keywords: PET; PSMA; Prostate Cancer; Radiotherapy.